This invention relates to fluid-quantity measurement systems and methods.
The invention is more particularly concerned with fluid-quantity measurement systems and methods employing ultrasonic probes with multiple reflectors.
In ultrasonic fluid quantity measurement systems, an ultrasonic transducer is mounted at the bottom of a vertical tube or still well, which is filled with fluid to the same height as fluid outside the tube. Examples of ultrasonic fluid-gauging systems are described in U.S. Pat. No. 2,990,543, U.S. Pat. No. 3,214,974, U.S. Pat. No. 3,290,944, U.S. Pat. No. 3,394,589, U.S. Pat. No. 3,693,445, U.S. Pat. No. 4,063,457, U.S. Pat. No. 4,183,007, U.S. Pat. No. 4,532,406, U.S. Pat. No. 4,748,846, U.S. Pat. No. 4,896,535, U.S. Pat. No. 4,909,080, U.S. Pat. No. 4,928,525, U.S. Pat. No. 5,095,747, U.S. Pat. No. 5,095,748, U.S. Pat. No. 5,119,676, U.S. Pat. No. 5,121,340, U.S. Pat. No. 5,127,266, U.S. Pat. No. 5,301,549, U.S. Pat. No. 5,309,763, U.S. Pat. No. 5,357,801, EP 138541, GB 2185575, GB 2247753, DE 3330059, JP 69024, WO 9119191, WO 9214996, SU 821939. The transducer transmits bursts of ultrasonic energy upwardly to the fluid/air interface where it is reflected back to the transducer. By measuring the time of travel of the bursts of energy, it is possible to calculate the height of the fluid surface. The speed of sound varies according to changes in fluid density and temperature. In order to compensate for this, it is usual practice for the still well to include a number of reflectors spaced apart along its length. Because the height of these reflectors is known, the ultrasonic reflections they produce can be used to calibrate the system and improve the calculation of the height and volume of fluid. In many applications, however, such as aircraft fuel gauging, it is necessary to know the mass of fuel, so a measurement of density is needed. Because of the temperature stratification that can occur within the fuel tank, there can be appreciable differences in the density of fuel at different heights, making it difficult accurately to calculate the fuel mass. The accurate measurement of fuel mass is of considerable importance to aircraft operators because it enables reduced quantities of fuel to be carried without reducing safety. This in turn enables greater payloads to be carried, with increased profitability.